Many of today's companies depend upon telecommunication networks to carry out their daily business. Because such networks often carry a great deal of information, they must optimally be fast, reliable and have a very high bandwidth.
As telecommunication networks are required to be more extensive and faster, the use of optical networking and/or optical components is becoming more and more important. Optical networks have the potential to provide much greater bandwidth than traditional networks. As optical networks are optimized, critical attention is paid to every component of such networks, including, but not limited to, optical routing circuits for routing network signals. Optical routing circuits may include tunable optical wavelength filters.
Tunable optical wavelength filters, typically, comprise a resonant cavity. Resonant cavities are well known in the fields of electromagnetics and optics. One type of resonant cavity that has been studied extensively is that which supports whispering gallery modes. This type of resonant cavity has been employed, as displayed in the prior art apparatus of FIG. 1, in integrated optics in the form of a cylindrical disk or ring cavity placed adjacent to coupling waveguides which are separated by a small spacing or gap. In use, a first (i.e., input) waveguide of the apparatus serves as the source of optical radiation. Only particular frequencies, or longitudinal modes, couple from the input waveguide to the cavity and then couple from the cavity to a second (i.e., output) waveguide. Thus, specific frequencies may be dropped from an optical input channel onto an optical output channel with high efficiency, a feature which makes this type of device useful for DWDM telecommunication applications such as add/drop filters, multiplexers, demultiplexers, and routers. In addition, these devices exhibit general resonance characteristics, which may be employed to advantage in dispersion compensation, electrically controlled switching, and enhancement of nonlinear optical effects. The usefulness of these devices increases when the resonance frequencies may be dynamically changed, allowing tunable wavelength filtering, tunable dispersion compensation, and other applications, and when large numbers of devices may be employed in the implementation of complex integrated optical circuits. Because of the long effective path length that is folded within the small cavity, these devices may also serve as modulators with very low drive voltage swing, and therefore, very low power consumption, and as nonlinear optical devices. Therefore, these devices can be considered a universal component of optical integrated circuits.
The prior art devices have, however, a few disadvantages and/or shortcomings. First, many of the prior art devices utilize side coupling, with the cavity and waveguides lying in the same plane. The manufacture of such a side coupled device requires very precise and high resolution lithography to be performed in order to control the gap spacing, which strongly affects the coupling between the cavity and waveguides, and therefore affects the quality factor, Q, of the cavity as well. Second, because the cavity and waveguides of such prior art devices are in the same plane, they are typically manufactured from the same material and have the same thickness. As a consequence, it is difficult to maximize performance of such prior art devices by phase matching the waveguides and cavity. Third, electrode placement in the planar prior art devices may be limited due to the side coupling arrangement of the waveguides and cavity and, hence, may interfere with the modes of the waveguides and, thereby introduce a potential source of loss. Fourth, the resonant cavities have only been considered as lying in the same plane, thereby limiting circuit flexibility and the density at which optical integrated circuits may be made. Finally, it is difficult to match the modal profile of the coupling waveguides to the desired mode of the cavity when using side coupling.
Therefore, there exists in the industry, a need for tunable resonant optical wavelength filters and other resonant devices for optical fiber telecommunication networks that may be manufactured in integrated optical circuits, and which address these and other related, and unrelated, problems.